Bower for most static assets

As per #359, this removes all checked-in static assets, and updates `bower.json` (and `.travis.yml`).

This hasn't yet added a submodule of the dependencies: I wasn't sure if that was an approach that was wanted. Do we want to check in `nbviewer/static/components` as a submodule? If so, in this repo (on some permanently-maintained branch), or another repo?

Related to this, the `Dockerfile`, I'm not much a a docker expert. Seems there are several options:
- submodule, as above
- install the whole `node/npm/bower` toolchain into the container 
- second container, do the provision there, then copy it over

Finally, depending on how much we're committing to having asset-oriented toolchain, this could then also be the time to add builds... though this might be yet another PR. Anyhow, LESS and requirejs would suggest themselves, I suppose, and significantly cut down the number of served requests.
I guess `bootstrap-collapse.js` and `bootstrap-collapse.js` are included in `bootstrap.min.js`... am I right?